The present invention relates to gas turbine nozzles repaired by welding, a gas turbine using the nozzles, a Co-base alloy suited as welding material for repairing the nozzles, and welding material.
A gas turbine nozzle (stationary blade) has a complicated shape. Therefore, the nozzle is manufactured by precision casting. However, since a nozzle with a defect or defects occurred during the precision casting is exposed to a high temperature combustion gas flow and subjected to large constraint force due to its construction, a crack or cracks occur in some cases while the gas turbine is working. Such a defect or defects occurred during the precision casting or a crack or cracks occurred during working of the gas turbine is repaired by tungsten inert gas arc welding, using a welding material (metal added to a welding portion during welding) by which a metal weld of an expected composition is formed after the welding for repairing. The repaired portion by welding is desirable to have the same high temperature characteristics as the nozzle.
As an alloy for gas turbine nozzles, an alloy, comprising, by weight, 0.20-0.30% C, 0.75-1.0% Si, 0.4-1.0% Mn, 24.5-30.5% Cr, 9.5-11.5% Ni, 6.5-8.0% W, not more than 2% Fe, 0.005-0.015% B, and balance Co (an alloy of Table 1, No.8) is used in many cases. Further, in order to improve various high temperature characteristics, various alloy compositions are invented. For example, JP A 61-546 discloses high strength Co-base heat resistant alloys, one of which comprises, by weight, 0.01-1% C, 15-40% Cr, 0.01-2% Si, 0.01-2% Mn, 5-15% Ni, 2-12% of at least one of Mo and W, 0.01-3% Al, 0.05-5% Hf and and balance Co, and another comprises further, in addition to the composition, 0.01-5% Ta and 0.005-0.1% Zr. Those alloys are for improving high temperature strength and oxidation resistance at the same time. JP A 7-300643 discloses a heat resistant Co-base casting alloy which comprises, by weight, more than 0.45% and not more than 0.8% C, 15-30% Cr, 5-15% Ni, 3-10% W, 1-5% Re, 0.01-1% Ti, 0.01-1% Nb, 0.01-1% Zr, not more than 1% Si, not more than 1% Mn, not more than 1.5% Fe, further at least one kind of element selected from 1-5% Ta, 0.5-5% Hf, not more than 0.1% B and 0.05-0.5% Al when desired, and balance Co and inevitable impurities, and gas turbine stationary blades (nozzles) for which the alloy is used. Further, similarly, JP A 7-224337 discloses a heat resistant Co-base casting alloy which comprises, by weight, 0.05-0.45% C, 15-30% Cr, 5-15% Ni, 3-10% W, 1-5% Ta, 0.01-1% Zr and balance Co, and gas turbine stationary blades (nozzles) for which the alloy is used. However, in a case where an alloy of the same composition as that of those nozzles is used as a welding material, drawing it into a ire is impossible because the alloy has such composition that forging is almost impossible. Therefore, as a conventional welding material, a welding material of composition with which a forging property weighs was used even though such a property that has the same high temperature characteristics as the nozzle body is a little sacrificed. For example, a Co-base alloy for forging (C:0.10, Cr:20, Ni:10, W:15, and balance Co (Table 1 alloy No. 6) is used as a filler (welding) material, or the filler material is used which is the above-mentioned nozzle alloy in which the carbon content is reduced from 0.23% to 0.13% to make easy forging, and which comprises, by weight, 0.13% C, 29.82% Cr, 10.22% Ni, 6.77% W and balance Co (Table 1 alloy No. 7). Further, WO 97/10368 discloses an alloy which comprises, by weight, 0.11-0.20% C, 20-30% Cr, 15-22% Ni, 5-15% Ta, 0.05-0.7% Zr and balance Co (Table 1, alloy No. 5), and further JP A 4-221035 discloses an alloy which discloses, by weight, 0.03-0.10% C, 24-32% Cr, 14-22% Ni, 2-8% Ta, 0.02-0.75% Ce and balance Co.
In recent years, in order to improve the power generation efficiency, an inlet temperature of a gas turbine is raised, so that a combustion temperature becomes higher and the nozzles are exposed to more severe temperature conditions than before. In combined cycle power generation, the nozzles receive severe stress hysterisis that thermal stresses due to repetition of starting and topping and steady stresses during operation are superimposed. Therefore, welding material also is required to have higher creep strength and more excellent thermal fatigue characteristics than before. However, particularly, although welding material for repairing nozzles is desirable to have a high temperature characteristic of the same level as the parent material, since the welding material is necessary to be such alloy composition that forging is possible, taking it into consideration to make a wire for a filler rod, and it should be excellent in weldability, the alloy No. 6, No. 7, etc. have been used as the filler material in many cases. However, the conventional welding rods have advantages and disadvantages. They are that brittle different phases occur in grain boundaries whereby thermal fatigue resistance is lowered, that precipitations aggregate and get coarsened whereby the creep strength is lowered, ever though it is excellent in creep strength and thermal impact characteristic, drawing into a wire is difficult, so that flaws remain on the surface of the filler rod, or that a cost increase is invited because of the difficulty of drawing it into a wire.